1. Field of the Invention
This invention relates generally to the field of methods and apparatus for drilling a wellbore along a desired trajectory. More specifically, the invention relates to tools for controlling the direction of a wellbore while drilling by rotating a drill pipe from the earth's surface is in progress.
2. Description of the Related Art
Wellbores used for petroleum production are often drilled along trajectories other than vertically from the earth's surface in a process referred to as directional drilling. The main purpose of directional drilling is for the wellbore to penetrate earth formations at a subsurface location different from the surface location from which the wellbore is started.
Various tools are well known in the art for directionally drilling wellbores, including hydraulically powered motors which turn a drill bit by converting the flow of drilling fluid ("mud") into rotational energy, the mud flow otherwise being used to cool the drill bit and lift drill cuttings out of the wellbore. Typical motors designed for directional drilling purposes include a housing which is bent at a preselected angle. These are known in the art as "bent-housing" motors. The power generation section of such motors is coupled to an output shaft, which ultimately turns the bit, by a flexible coupling. When this type of motor is used to adjust the trajectory of the wellbore, the entire drilling assembly, which includes drill pipe, drill collars, the motor, stabilizers and the drill bit, is slowly rotated from the earth's surface by a rotary drilling rig or similar apparatus so that the bend of the motor housing is oriented in the direction towards which the wellbore trajectory is to be adjusted. As is well known in the art, after the desired trajectory adjustment to the wellbore is finished, the bent-housing motor must be removed from the drilling assembly. This requires a time-consuming "trip out of the hole", where the entire drilling assembly is removed from the wellbore and a different assembly, which may exclude the bent-housing motor, is inserted into the wellbore to continue drilling along the adjusted trajectory.
In other cases, a so-called "steerable" motor can be used both to adjust and to maintain the trajectory of the wellbore during drilling. The typical steerable motor has a bent housing as does the bent-housing motor, but the bend is much smaller in magnitude. Adjusting the trajectory of the wellbore is accomplished with a steerable motor by adjusting the orientation of the motor housing as is done for the bent-housing motor, but when the desired trajectory is achieved, the trajectory can be maintained by rotating the entire drilling assembly from the earth's surface. Rotating the housing of a steerable motor generally causes the existing trajectory of the wellbore to be maintained.
Limitations of mud motor-based directional drilling include limited life of the power-generation section of the typical motor, which includes a positive displacement rotor disposed inside an elastomeric-lined stator. An additional limitation is that orientation of the motor housing can often be difficult to maintain, because as the drill bit contacts the earth formations to drill them, a reactive torque is generated against the motor housing which changes the orientation. A particular limitation of directional drilling using steerable motors is that steerable motors tend to drill a "corkscrew" shaped hole where the motor housing is rotated to maintain trajectory of the wellbore.
A different type of steerable rotary tool for directional drilling is presented in U.S. Pat. Nos. 5,484,029 and 5,529,133 to Eddison and U.S. Pat. No. 5,617,926 to Eddison et. al, hereafter collectively referred to as Eddison. This steerable tool comprises an upper housing which connects to the drill pipe and a lower driveshaft which attaches to the drill bit. The housing and driveshaft are coupled so that rotary torque from the housing is transmitted to the shaft while allowing the rotational axis of the bit to pivot universally to a limited degree relative to the longitudinal axis of the housing. Enclosed inside the housing is an internal "eccentric weight" arranged to have relative rotation with respect to said housing. Due to the effects of gravity, the weight remains substantially stationary at the low side of the directional wellbore. The upper end of the driveshaft is coupled to the stationary weight through an eccentric bearing to maintain the bit axis pointed in only one direction as the bit is rotated. Additionally, Eddison discloses an intricate clutch system used to alter the orientation of the drill bit downhole and a measuring-while-drilling (MWD) tool for monitoring directional parameters with respect to the position of the weight.